1. Field of the Invention
The invention relates to a process and apparatus for the preparation of plastic parts or objects from at least two reactive synthetic components. The reactive synthetic components are brought into contact in a mixing chamber by way of impinging streams from injection ports. An additional input port supplies an auxiliary component or components to the mixing chamber. An example of an auxiliary component is a liquid foaming agent for the production of cellular foam.
2. Description of the Related Art
Impingement mixing devices are known for producing plastic parts or objects and are frequently referred to as mixing heads or reaction injection molding mixing heads. Such heads are described in U.S. Pat. Nos. 3,706,515 and 3,975,128. A mixing device for mixing together multicomponent plastic materials, such as polyurethane, and foaming such materials including a supply of air to the mixing chamber is described in U.S. Pat. No. 4,053,283. In the preparation of cellular foam such as polyurethane from two or more reactive synthetic plastic components it is also known to utilize a liquid foaming agent, for example Freon 11. The liquid foaming agent is mixed with one of the synthetic components, for example polyol, prior to its mixing with the rest of the components. The foaming agent passes into the gaseous form under the effect of the heat generated by the reaction of the synthetic components in the casting mold, thereby producing a cellular structure. Upon the cooling of the cellular foam the foaming agent returns to the liquid state. If the cell walls are sufficiently stable, atmospheric air gradually enters the cell structure, whereby the cells are filled with air. In the case of very thin cell walls, without additional measures the cellular structure would shrink. In such cases, an inert gas, for example compressed air, is added to the mixture of synthetic substances. It immediately fills the cells and prevents shrinking. However, the inert gas, in contrast to the liquid foaming agent, cannot be added to one of the reactive synthetic components prior to the mixing process under an adequately high pressure. This is because the mixing of a highly compressed gas into the liquid component cannot be metered with the necessary accuracy. Furthermore, the necessary compression of the inert gas (to more than 200 bar) requires a substantial investment in equipment, which under realistic operating conditions cannot be justified.
In the system disclosed in U.S. Pat. No. 4,053,283 for the above reasons, the inert gas is introduced through an axial channel of a discharge piston into the mixing chamber, which has a pressure of about 5 bar during the mixing phase. The inert gas may be mixed in at a correspondingly low pressure of approximately 6 to 7 bar. At that pressure the gas is easily metered. The conical outlet orifice of the channel in the frontal surface of the piston may be closed by a conical closure piece, to prevent the entry of mixed synthetic plastic material from the mixing chamber into the axial channel. The closure piece is actuated by a pin which passes through an axial channel. Discharge pistons normally have small dimensions for example 5 mm diameter and 200 mm in length and may be equipped with axial recirculating grooves for the synthetic plastic components. The axial channel and the pin guided therein therefor can have only a very small diameter of for example 0.8 mm, the production of which is difficult and results in major problems in production technology. Furthermore, despite the conical closure piece, the entry of mixed plastic material into the axial channel cannot be completely prevented. An increase in pressure in the mixing chamber during the discharge of the mixture (to about 40 bar) is a result of a restriction of the outlet of the mixing chamber. The increased pressure acts on the frontal surface of the discharge piston during the entire discharge process. The sealing effect of the conical surfaces between the closure piece and the channel outlet is not sufficient to prevent penetration of the highly compressed synthetic plastic material. The penetrating material cannot be flushed out by the inert gas due to its strong adhesion to metal surfaces and therefore hardens in the axial channel. In view of the already very small cross section of the channel this rapidly leads to complete clogging of the axial channel. In actual practice, the axial channel must be cleaned in intervals of approximately 4 hours, requiring dismantling of the hydraulic unit and removal of the discharge piston.
U.S. Pat. No. 4,115,299 shows a "frothing" process and head where a liquid foaming agent with a low boiling point is introduced in the quieting chamber of an L-shaped mixing head. The mixing chamber of the head opens into a quieting chamber mounted at a right angle to it. The foaming agent is injected against the opening of the mixing chamber in a counter current. In the process, the liquid foaming agent expands and as a result of its low boiling point, passes into the gaseous state, whereupon a plastic foam is formed in the quieting chamber. No addition of an inert gas is provided for in the process according to U.S. Pat. No. 4,115,299. In this case, mixing of inert gas in the quieting chamber would not lead to satisfactory mixing, even if in place of a foaming agent with a low boiling point a normal foaming agent mixed into one of the components were used.